On Wednesday, the PEW Center on the States released a study centered around a surprising fact: states now spend $51 billion a year on corrections, with most of that money going to prisons. The culprit, the report says, is longer prison terms, which have doubled since 1990.

In California, for instance, the average prison term went up 51 percent between 1990-2009, with inmates serving an average of 2.9 years.

Calculating the cost of longer sentences nationwide, PEW found that, “for offenders released from their original commitment in 2009 alone, the additional time behind bars cost states over $10 billion, with more than half of this cost attributable to non- violent offenders.

Meanwhile, crime has dropped across the country, a fact often cited as evidence of the effectiveness of harsh prison sentences. According to the PEW report, “increased use of incarceration accounted for one-quarter to one-third of the crime drop in the 1990s,” and, the report suggests, the country’s reached a “tipping point” where relying on more incarceration will no longer improve results.

Particularly for non-violent offenders, PEW found, “there is little or no evidence that keeping them locked up longer prevents additional crime.”

States, the study says, have rightly begun to look towards more cost-effective alternatives.